How To Calculate Battery Run Time
There are just too many questions you will wonder when designing your device with a battery inside it.
The boss just want a cheap and small battery with it, but with no further information on how long does the end customer wants, how small can it be.
in this article, we will show you:
1 how to calculate the run time of a specific battery?
2 how much capacity can I get given a specific battery size requirement?
3 Online Lipo battery Size-Capacity Calculator
4 How to convert Watts to Amps or Amps to Watts or Volts to Watts
Ready for your battery design?
In the ideal/theoretical case, the time would be Time(H) = Capacity(Ah)/Current(A).
If the capacity is given in amp-hours and current in amps, time will be in hours (charging or discharging).
Let’s see an actual case, 10 Ah battery delivering 1A, would last 10 hours. Or if delivering 10A, it would last for only 1 hour, or if delivering 5A, it would last only for 2 hours.
In other words, you can have “any time” as long as when you multiply it by the current, you get 10Ah (the battery capacity).
It is that simple.
So for a 18650 2500mAh(2.5Ah) battery with a device that draws 500mA(0.5A) you have:
Please take note that most batteries, especially those with circuits, will not work down to 0 Volts as a power supply (if it goes to zero, it will have shorter battery life, or even become dead battery if not charged in time), that’s to say,your circuit will stop working at a set voltage before the battery is fully drained.
We, therefore, will need to times 0.8-0.9 for the calculation:
that’s 2.5Ah/0.5A*0.9=4.5 Hours
What if you know Watts only, you will notice that every device use watt to determine it’s main specifications.
A 5 Watt bulb, 20W Laptop, 100W Motor, 200W Solar Street Light, you just name it.
In Theory, that’s:
Discharging Time=Battery Capacity*Battery Volt/Device Watt.
Say, 5AH*3.7V/10 Watts = 1.85 hrs
With 90% Power efficiency for Li-ion/LiPo batteries. Then
Discharging Time=Battery Capacity * Battery Volt*0.9 / Device Watt
5Ah*3.7V*0.9/10W = 1.66 hours
Let’s explain with more examples:
for a 1800mAH 3.7v 18650 battery to power a 3.7V 10W digital device, how to calculate the running time?
for 3.7V 10W device，working current would be 10÷3.7 = 2.7027A = 2702.7 mA
In theory that’s: 1800mAh ÷ 2702.7 mA = 0.666 h = 40 min
In reality that’s: 1800mAh ÷ 2702.7 mA*0.9 = 0.599h = 36 min
Notes: 1A=1000mA mA is current, mAh is Capacity
Or you can use 3.7V*1.8Ah(1800mAh)*0.9/10W=0.599h=36min
Another example: 12V 60Ah battery pack to power 220V 100W light
Working time: 12V*60Ah*0.9/100W=6.48 H